METABOLISM 279 
matters are often deposited in the substance of cell-walls. 
The oxalate of calcium occurs frequently in this situation 
(fig. 127). In other cases it is deposited in special cells, 
where it forms clusters of crystals of characteristic shape 
(fig. 128, a, B). In these cases the cluster of crystals is 
usually invested by a delicate skin derived from the proto- 
plasm, thus shutting it off completely from any participa- 
tion in the metabolism of the cell in which it lies. 
Carbonate of calcium may also be deposited in the sub- 
stance of the cell-wall, or of protrusions from it, as in the 
eystoliths of Ficus, Urtica, and other plants (fig. 129). 
Fic. 129.—SEction oF Portion oF 
Lear or Ficus, sHowIna Cysto- 
Fria. 128.—Crystats oF OXALATE oF LITH (cys) IN LARGE CELL OF THE 
Catctum. a, From Brnr (Sphera- THREE-LAYERED EPpiDERMIS (ep). 
phides); B, FROM ARum (Raphides). (pa) PaLIsaDE Layer, 
Silica again is accumulated in’ the epidermis of many 
grasses, and of the horsetails (Hquisetum). 
Though many of these substances, both excretions and 
bye-products, are of no value for nutrition, some of them 
may play a very important part in the defence of plants 
against their natural enemies, their nauseous smell or 
flavour preventing their being eaten by animals, &c. Some 
odours and the nectar found in flowers are doubtless of 
great service in attracting insects, which assist in the process 
of cross-pollination, to be discussed in a subsequent chapter. 
Though we cannot trace the formation of all these various 
substances, both bye-products and excretions, directly to 
the self-decomposition of the protoplasm, but must regard 
